nixosTests.installed-tests.flatpak: Fix
nixosTests.installed-tests.flatpak: Fix

Along with the `socat` fix in the parent commit, this makes
the Flatpak’s installed tests finally pass again.

The tests seem to need slightly over 2G of disk space,
and it appears that the test suite was ported to Python 3 in 1.5.1:
https://github.com/flatpak/flatpak/commit/2b6641575db75d8af8228749798f4ea635797577

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs